A number of different access methods an which wireless cellular communication networks are based are known. In a typical wireless cellular network, the area covered by a cellular network is divided into a plurality of cells or cell sectors. Each cell is served by a base station which transmits signals to, and receives signals from, mobile stations located in the cell or cell sector associated with the respective base station. In a system using antenna arrays with analogue or digital beamforming, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in one or more beam directions from which a signal from the mobile station is received. In other words, a signal intended for a given mobile station is not transmitted throughout the cell or cell sector but rather only to the part of the cell or cell sector in which the mobile station is located.
As the signal, which is transmitted by the base transceiver, station is usually only be transmitted in a particular direction, the resulting transmission may have a relatively narrow angular spread with the transmission power concentrated into that narrow angular spread. This results in a better signal to noise ratio. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver can be achieved. The interference caused to other mobile stations in the same cell or adjacent cells by the signal transmitted by the base station to the mobile station is also reduced. This increases the capacity of the system and/or increases the quality of communications.
A signal from the base transceiver station to a mobile station will generally follow several different paths. Those pluralities of paths are generally referred to as multi-path. As each multi-path follows a different route from the base transceiver station to the mobile station, the length of each multi-path differs. Consequently a signal following a first multi-path will arrive at the mobile station at a different time from a signal following a second multi-path. The signals received via each of the multi-paths may be out of phase with respect to each other at the mobile station. If the signals are out of phase with respect to each other, it is possible that these signals may interfere with each other destructively, thereby reducing the signal to noise ratio of the transmitted signal transmitted by the base station: In other words, due to this destructive interference, the signal may not be received successfully by the mobile station.
It has been proposed to use beam diversity in systems using beamforming techniques to address the difficulties caused by multi-path interference in such communication system. Examples of such beam diversity schemes are discussed in PCT/EP99/03093. Beam diversity involves transmitting the signal from the base transceiver station to the mobile station in more than one beam direction using for example a main beam and a diversity beam. The principal is that it is unlikely that both the main and diversity beams will suffer severe multi-path interference for a given signal. The mobile station is then arranged to select for processing the signal having the largest signal to noise ratio, or to combine the signal from both the main beam and diversity beam, resulting in an improved overall signal to noise ratio.
One of the access methods which is used in the IS-95 standard of the USA and which is currently proposed for the next generation standard is code division multiple access (CDMA). In CDMA mobile stations are distinguished by the spreading codes which they apply to the information to be transmitted. Likewise, signals intended for a given mobile station from a base station can be identified by the mobile station from the spreading codes applied to the signals. As a mobile station moves from one cell to another “soft handoff” is possible. Soft handoff is where a mobile station is in communication with two or more base stations at the same time. Typically, this will occur when a mobile station is in the region of two cells.
It is possible that a large number of mobile stations may be in soft handoff at any one time. In some situations, it is envisaged that, for example, 50% of the mobile stations will be in soft handoff.
It is known that a cell which is likely to contain a large number of mobile users, for example, a cell located within a densely population urban area, can be sub divided into a number of sectors. Although the cell has a single base transceiver station, separate antenna arrays are provided such that each sector of the cell is associated with a particular antenna array. Effectively each cell sector is analogous to a cell except that the base stations for adjacent cell sectors are in a single location. In this situation it is possible for a mobile station to be located on or near to the boundary between different sectors, in an analogous fashion to a mobile station which is located on the boundary between two different cells. In this case, the mobile station may receive simultaneous signals transmitted from each antenna array associated with the respective neighbouring sectors. The base stations at the same location may share information received from different antenna arrays. This is referred to as “softer handoff”.
Clearly if a relatively large number of mobile stations are in soft or softer handoff, difficulties can arise if beamforming base stations are used as the number of beams transmitted by each base station may be increased. In CDMA systems code orthogonality is usually employed in the downlink signal from the base station to the mobile station in order to minimise the interference from other users in the same cell. However, this orthogonality is preserved only if the delay spread of the downlink channel is sufficiently small so that the channel can be characterised as a one-tap channel (this type of channel is often referred as a flat fading channel). This requirement means that in CDMA systems the delay spread must be much smaller than the chip duration. In one-tap channels the downlink diversity techniques give the best possible gains. In practice, especially in macro cells, the delay spread is often much greater than the chip duration and this leads to significant loss of orthogonality between the co-channel users. Such multitap channels already possess multipath diversity and therefore, increasing the number of downlink diversity beams from two to four does not produce significant excess diversity gain. On the contrary, increasing downlink beams in multitap channels severely destroys code orthogonality and results in performance loss. Accordingly, it is beneficial to reduce the number of downlink beams in soft or softer handover cases. It is therefore an aim of embodiments of the present invention to address this problem.